quad wont run right

Have a 350 yamaha as well as a 350 honda 4 wheeler, both 2006 models, rode easy, put up clean and now that it's warming up, can't get them started. drained all fuel, changed oil and plugs, the yamaha will start and idle and then cuts off after it idles for about 25-35 seconds, or immediately if I give it any throttle at all. the Honda wont start at all. Tried turning off fuel, dry firing approx 20 times, adding fuel in spark plug hole, reassembling and it will fire and shut off immediately after it starts, turn fuel back on and NOTHING. I'm thinking it may be a carburater problem. Any help would be appreciated, the Yamaha is most important to me right now, just need to get it running right. Thanks in advance.

You remove the plug, turn off the fuel and hold the throttle fully open with the choke off to purge the fuel vapors from the combustion chamber. This gets rid of any old fuel in the chamber. Then you add new fuel directly into the chamber in a small amount to start the engine. Once it starts you're essentially back to a clean combustion chamber. Then you can go back to a basic starting procedure. Old fuel is usually a problem in what you're describing.

You have some plugged intermediate jet circuits in the carburetor that will have to be opened. Those are probably because of the old fuel.

You can dismantle the carb to clean it, but you can try to clean it on the engine. To do that, first get an good aerosol carb cleaner from an auto parts store. Don't use the type you pour in the fuel tank.

Turn off the fuel from the tank and disconnect the fuel line from the tank. If you have a drain on the carb float bowl that you can get to for draining remove it, drain the fuel and reinstall. If not, with the spark plug in place, and the choke fully on, and throttle at half , spin the engine over about ten times or so. This will pull the fuel from the float bowl and make room for the cleaner. Spray cleaner through the fuel line and fill the carb bowl as full as you can.

Then, with the choke fully on and the throttle closed, spin the engine over about five times. This will pull the cleaner up through the carburetor. Let it sit for about five minutes, then repeat and let it sit for about half an hour.

Then pull the plug, purge the fuel/cleaner from the engine with full throttle and no choke/no plug, add a small amount of fuel (teaspoon) through the plug hole and start the engine. Then, with good fuel in the tank hook up the fuel line and start the engine. If it still doesn't run right, but you can get it to idle ongoing, spray carb cleaner into the fuel tank in a high concentration and let it continue to run. It may clean itself out.

If not and you still have a problem, you'll have to dismantle the carb for a complete cleaning.

For future reference, use fuel stabilizer in the tanks for extended idle periods.